Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellowness, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number.
A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Therefore, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, the most representative method is a method using an episulfide compound (see Patent Document 1).
Meanwhile, it has been also studied to add a high value to plastic lenses by imparting various functions thereto, and in particular, many lenses to which photochromic function is imparted have been proposed to be used for spectacles. As techniques of imparting photochromic function, a technique of coating a lens surface with a composition containing a photochromic compound (hereinafter referred to as “the coating technique”, see Patent Documents 2 and 3) and a technique of molding a lens, wherein a photochromic compound is mixed with a composition for optical materials (hereinafter referred to as “the mixing technique”, see Patent Documents 4 and 5) have been mainly proposed.
However, when using the coating technique, the manufacturing process is complicated, the yield may be reduced, and peeling may be caused during use, and regarding the mixing technique, since the photochromic compound reacts with a monomer of isocyanate or the like, the mixing technique has been successfully applied only to lens materials having a low refractive index.
Application of the mixing technique to materials having a high refractive index has been also proposed, but the highest value of the refractive index obtained was 1.60 (see Patent Documents 6-8).